The present invention relates to commutators for electrical machines and to methods of manufacture of the commutators. More specifically, the invention relates to a commutator and method of manufacture in which a lamination assembly is held together by shrink rings.
In DT-OS 2 062 864 a commutator is described which consists of a lamination assembly held together by shrink-rings. The shrink-ring which serves to support the commutator comprises two rings with an elastic connecting portion. One of the rings is shrunk on to the commutator and thus serves to hold the lamination assembly together. The commutator hub is shrunk on to the other ring, which is joined to the first ring by an elastic connecting portion. This arrangement allows thermal expansion of the commutator in the axial direction and within certain limits permits changes in the radial direction, without losing a tight fit on the shaft or on the commutator hub.
Because shrink-rings have to meet exacting requirements with regards to strength, and because of the special shaping and the hardness of the material a shrink-ring for supporting a commutator is very costly to make. Also, a method of manufacture which includes the step of assembling the commutator by means of two different shrinking operations with one after the other results in long assembly times, and hence high costs. It has been attempted to make the shrink-ring and the commutator hub in one piece, but this solution does not result in a combination of low production costs and high material strength.
An object of the present invention is to provide a commutator for electrical machines which permits high mechanical stresses and is distinguished by ease of installation, and hence low production costs.
Another object of the invention is to provide a method of manufacture for the commutator for electrical machines of the present invention.
The commutator of the present invention may be manufactured by having the ring portions of the shrink ring which serves to support the commutator simultaneously shrunk on to the commutator and onto an outside surface of the commutator hub in single operation.
In other words, the chief advantage of the invention lies in providing a commutator in which both shrink-fit joints can be made in one operation.
The invention concerns a commutator and a method of manufacture of the commutator for electrical machines which consists of a lamination assembly that is held together by a pair of shrink rings. One of the shrink rings serves to support the commutator on a commutator hub and comprises first and second ring portions having between them a decoupling portion. The first ring portion holds together the lamination assembly. The second ring portion is secured to the commutator hub. The other of the two shrink rings holds together the lamination assembly.
Of particular benefit is an arrangement whereby the ring portions of the one shrink ring have essentially the same inside diameters and outside diameters. It is then possible to produce the shrink ring by rolling, in which case only slight subsequent machining is necessary and thus savings in material and time are achieved. It is of advantage to locate the decoupling portion at the outside diameter of the ring portions since with this configuration not only are material stresses during operation of the commutator greatly reduced, but also a greater resistance to sparkover is achieved by virtue of a larger insulating distance. To define the distance between commutator and commutator hub it is convenient to provide the commutator hub with a stop on the surface receiving the ring portion. It is of advantage to provide the commutator hub with air passages so as to ensure good cooling of the commutator. The shrink-rings are preferably of steel having high tensile strength, preferably .sigma..gtoreq.70 kp/mm.sup.2, and the commutator hub is preferably of grey iron. A construction which is relatively inexpensive but able to withstand very high stresses is obtained in this way. To increase the resistance to sparkover it is advantageous to provide insulating material at the inside and outside of the decoupling portion. A castable, curable synthetic resin is particularly suitable as the insulating material. To reduce the volume of cast insulation it is advantageous to attach an insulating ring by way of spacers to the end surface of the commutator facing the commutator hub. The cavity between the commutator hub, the commutator and the decoupling portion is then preferably filled completely with resin.
The method of manufacturing a commutator according to the present invention is such that after the ring serving to support the commutator is heated, one ring portion is introduced into a slot in the lamination assembly and the commutator hub is then inserted into the other ring portion. Direct mechanical bonds with the lamination assembly and the commutator hub are then obtained by cooling the two ring portions simultaneously.
It is of benefit to attach the insulating ring to the end surface of the commutator facing the commutator hub before the one ring is shrunk on to the lamination assembly and the commutator hub.
It is also advantageous to fill the space between the commutator and the upper part of the commutator hub resulting from the shrinking operation with a curable synthetic resin, at least along the decoupling portion. To avoid air inclusions it is beneficial to pour the synthetic resin while the commutator is rotating.